Mine are 14.5" .
You haven't ask, but I will offer this. I tried the tuning plates for three or four cooks and took them out. They were more trouble and mess than they were worth.
My Brazos will run just fine end-to-end without them. I put a 16" loaf pan across the FB end of the cook chamber below the cook grate , to use as a water bowl. I will add pic of gauges.
Sometimes my temps end-to-end are even, sometimes they vary by 25* either way. I don't think it matters. I just move my meats around about every hour.
For a while, I was using the FB door to regulate temps end-to-end. If the door was about 2" open, the increased air flow would throw heat to the stack end of the cook chamber. But that ended up just being another variable I had to deal with in keeping steady cook temps. I now leave the door shut and the baffle wide open.
With the tuning plates, my smoker becomes something of a reverse flow smoker. I get the juices dripping down on the plates creating that flavor of burning fat. Some like that, its not something I shoot for.